It is known (refer to “Optics” (Volume 27, No. 1, 1998, Pages 12-17) written by Hisao Kikuta and Koichi Iwata and issued by the Optical Society of Japan) that a corrugated pattern forming sheet in which a corrugated pattern composed of minute wavelike irregularities is formed on a surface and the average pitch of the corrugated pattern is below the wavelength of visible light can be utilized as optical elements, such as an antireflector and a retardation plate.
Here, the average pitch is an average value of the pitch that is the distance between an apex of a convex portion of the corrugated pattern and an apex of a convex portion adjacent to the convex portion in a case where the corrugated pattern runs only in one direction. On the other hand, the average pitch is obtained as follows in a case where the corrugated pattern does not run in a specific direction. First, the top surface of a corrugated pattern is captured by an atomic force microscope, and the image is converted into a gray-scale file (for example, tiff format, etc.).
In an image (refer to FIG. 4) of the gray-scale file, a lower whiteness means that a bottom of a concave portion is deeper (a higher whiteness means that an apex of a convex portion is higher). Next, the image of the gray-scale file is Fourier-transformed. The image after the Fourier transformation is shown in FIG. 5. In the image after the Fourier transformation, the direction as seen from the center of a white portion represents the directivity of the gray-scales, and the inverse number of the distance from the center to the white portion represents the period of the gray-scale image. In a case where the corrugated pattern does not run in a specific direction, an image showing a white circular ring like FIG. 6 is obtained. Next, the luminance (Y-axis) with respect to the distance (X-axis) from the center of the circular ring is plotted by drawing a linear auxiliary line L2 from the center of the circular ring in the image after the Fourier transformation towards the outside (refer to FIG. 4). Then, a value r on the X-axis showing a maximal value in the plot is read. The inverse number (1/r) of this value r is an average pitch.
The corrugated pattern forming sheet can be utilized as an antireflector on the basis of the following reasons.
In a case where the corrugated pattern is not provided in a sheet surface, reflection is caused by an abrupt change in refractive index in an interface between the sheet and air. However, in a case where a wavelike corrugated pattern is provided in a sheet surface, i.e., an interface between the sheet and air, a value (hereinafter referred to as “middle refractive index”) between the refractive index of air and the refractive index of a corrugated pattern forming sheet is shown in a portion of the corrugated pattern, and moreover, the middle refractive index continuously changes in the depth direction of the corrugated pattern. Specifically, the refractive index of a deeper position approaches the refractive index of the corrugated pattern forming sheet. Because the middle refractive index continuously changes in this way, reflection of light can be suppressed without causing an abrupt change in refractive index in the interface as described above. Further, if the pitch of a corrugated pattern is below the wavelength of visible light, coloring by diffraction of visible light, i.e., interference of visible light, in a portion of the corrugated pattern is hardly caused.
Further, the corrugated pattern forming sheet can be utilized as a retardation plate because a portion of the corrugated pattern shows optical anisotropy against the light that is incident on the corrugated pattern with the result that air and a corrugated pattern forming sheet whose refractive indexes are different from each other are arranged alternately. Moreover, if the pitch of a corrugated pattern becomes approximately equal to or less than the wavelength of visible light, a phenomenon in that the same retardation is shown in a broad visible light wavelength range will appear.
As a specific example of such a corrugated pattern forming sheet, for example, a sheet in which gold is vapor-deposited on one surface of a sheet made of heated polydimethyl siloxane to form a metal layer, and then cooled, whereby the sheet made of polydimethyl siloxane is made to shrink, thereby forming a wavelike corrugated pattern in the surface of the metal layer, is suggested in “Nature” (No. 393, 1998, Page 146) written by Ned Bowden.
Further, a sheet in which a foundation layer and a metal layer are sequentially formed in the surface of a heat-shrinkable synthetic resin film, and then the heat-shrinkable synthetic resin film is made to thermally shrink to form a wavelike corrugated pattern in the surface of the metal layer is suggested in JP-A-Sho63-301988.
A sheet in which a layer made of a material that is reduced in volume by exposure is formed, and the layer is exposed to form irregularities in a surface thereof is suggested in JP-A-2003-187503.
However, none of the corrugated pattern forming sheets described in JP-A-Sho63-301988, JP-A-2003-187503, and “Nature” written by Ned Bowden show excellent performance as optical elements. Specifically, when the corrugated pattern forming sheets are used as antireflectors, the reflectance cannot be made low enough, and when the corrugated pattern forming sheets are used as retardation plates, the retardation cannot be made large enough, and the same retardation cannot be caused over a broad wavelength range.
Further, photolithography by the visible light that uses a pattern mask is known as a method for manufacturing a corrugated pattern forming sheet. However, a corrugated pattern forming sheet with a pitch below the wavelength of the light that can be utilized as an optical element cannot be manufactured by this method. Therefore, it is necessary to apply an ultraviolet laser interference method or electron beam lithography that allows finer processing. In these methods, a resist layer formed on a substrate is exposed and developed with ultraviolet laser interference light or electron beams to form a resist pattern layer, and irregularities are formed by a dry etching method, etc. by using the resist pattern layer as a mask. However, when the ultraviolet laser interferometer method or electron beam lithography is applied, there is a problem in that this method is not suitable for mass production because processing in a broad region which exceeds 10 cm is difficult.
A method for arranging a particle layer on a substrate and dry-etching the surface of the substrate by using the particle layer as an etching mask is also suggested in JP-A-2005-279807. However, there is a problem in that this method is also not suitable for mass production because processing in a broad region which exceeds 30 cm is difficult.